#define _CRT_SECURE_NO_WARNINGS


# include<stdio.h>
# include<math.h>
int main()
{
	double a, b, c, s, area;
	scanf("%lf %lf %lf", &a, &b, &c);
	if (a + b > c && a + c > b && b + c > a)
	{
		s = (a + b + c) / 2;
		area = sqrt(s * (s - a) * (s - b) * (s - c));

		printf("%lf\n", area );
	}
	else
	{
		printf("no");
	}
		return 0;
}